SAO PAULO -- Pope Benedict XVI began his first papal trip to Latin America stressing church opposition to abortion yesterday, suggesting that Catholic politicians in Mexico had excommunicated themselves by legalizing abortion in that nation's capital.

Benedict, who will inaugurate an important regional bishops' conference during his trip, also spoke strongly against abortion during his first speech in Brazil. Speaking in Portuguese, he said he's certain that the bishops will reinforce "the promotion of respect for life from the moment of conception until natural death as an integral requirement of human nature."

Thousands of faithful waited in the cold rain for a glimpse of Benedict, then chanted "Bento, Bento" and waved flags of different South American nations as he blessed them at the monastery where he is staying.

The Vatican says Benedict will confront major challenges during his visit, such as the church's declining influence in Brazil, the rise of evangelism, and efforts to expand access to abortions in the region.

Catholic officials have been debating for some time whether politicians who approve abortion legislation as well as doctors and nurses who take part in the procedure subject themselves to automatic excommunication under church law.

The pope was asked where he stands on the issue during the flight to Brazil, in his first full-fledged news conference since becoming pontiff in 2005.

"Do you agree with the excommunications given to legislators in Mexico City on the question?" a reporter asked.

"Yes," Benedict replied. "The excommunication was not something arbitrary. It is part of the (canon law) code. It is based simply on the principle that the killing of an innocent human child is incompatible with going in Communion with the body of Christ. Thus, they (the bishops) didn't do anything new or anything surprising. Or arbitrary."

Church officials later said the pope might have inferred from the question that the Mexican bishops had issued a formal declaration of excommunication for the legislators, something Mexican Cardinal Norberto Rivera has said he has no intention of doing.

Benedict's spokesman, the Rev. Federico Lombardi, said the pope was not setting a new policy and did not intend to formally excommunicate anyone -- a rare process that is separate from the doctrine of self-excommunication.

"Since excommunication hasn't been declared by the Mexican bishops, the pope has no intention himself of declaring it," Lombardi said in a statement approved by the pope.

But Lombardi added that politicians who vote in favor of abortion should not receive the sacrament of Holy Communion. Excommunication is the severest penalty the Roman Catholic Church can impose on its members. When someone is excommunicated "his status before the church is that of a stranger," the New Advent Catholic Encyclopedia says. In practical terms, that means the excommunicated person is forbidden from receiving the sacraments and participating in public worship.

Church teaching says anyone who has an abortion is automatically excommunicated. "Being a conspiring or necessary accomplice" to an abortion also means excommunication.

The Mexican politicians who supported the measure shrugged off Benedict's comments yesterday. "I'm Catholic and I'm going to continue being Catholic even if the church excommunicates me," said leftist Mexico City lawmaker Leticia Quezada. "My conscience is clean."

On the plane from Rome, Benedict said the exodus of Catholics for evangelical Protestant churches in Latin America was "our biggest worry."

But he said that the spread of Protestantism shows a "thirst for God" in the region.